The present invention provides for processes for removing contaminants from gas stream emissions. More particularly, the present invention provides for removing contaminants such as nitrogen oxides from gas streams in sulfuric acid production processes.
Sulfuric acid is used in a wide spectrum of process industries. Sulfuric acid is believed to be the world's largest chemical produced. Over past few decades, worldwide, most of the sulfuric acid is produced by a contact process, which involves generating a sulfur dioxide containing gas stream from variety of sulfur sources. Examples include burning elemental sulfur, or process of roasting metal ore or burning H2S arising from industrial operations such as hydrodesulfurization of petroleum products or simply burning waste containing sulfate or sulfuric acid or combusting spent sulfuric acid all generate SO2 in the gas stream. If the source of sulfur is dirty, flue gas is conditioned and oxidized to convert almost all SO2 to SO3 over a V2O5 catalyst in a multi pass converter. The oxygen required for oxidation is either present or supplemented in the form of additional air or oxygen. This SO3 containing gas stream is absorbed in sulfuric acid solution, which results in the H2SO4 product as a >95% wt acid or oleum of desired strength.
Since sulfuric acid is a very low cost product, and reactions are exothermic, heavy emphasis is put on heat integration and therefore generally most exothermic heat that is recovered is used within the process for captive requirement of energy and any net surplus is exported in the form of steam. Nitrogen oxides (NOx) are generally formed during the SO2 generation step in varying quantities based on a variety of factors. When an SO3 containing gas stream is absorbed into sulfuric acid solution, some of the NOx reacts with a circulating solution of sulfuric acid forming a complex which is referred in industry as niter (nitrosyl sulfuric acid) and some of its homologs. Niter in the product is an undesirable impurity in many applications and also imparts some color to the product.
Some of the NOx which leaves the scrubber passes through much of the process equipment and is finally exhausted to the environment. It is often noted that the plume arising from the sulfuric acid production facility is correlated with SOx emissions, NOx emissions, niter, types of mist eliminating devices and various process parameters. Some of these environmental problems are alleviated in the modern plant by a dual stage absorption process, choosing effective mist elimination devices followed by a caustic scrubber. Selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) type of processes have been suggested for NOx removal. However, the problems of NOx emissions, acid plume, deterioration of product quality due to niter and nitrogen containing compounds in sulfuric acid still exists at varying levels in the industry. With increasing environmental concern and government oversight, the present levels of NOx controls are not adequate.
Sulfuric acid is a high production volume but a low cost and low margin chemical. The cost of a plant producing sulfuric acid is relatively high. The relationship of the capital cost and the plant capacity is not linear. Therefore, plants with a larger production capacity achieve much better scales of economy compared to plants with smaller capacities. Sulfuric acid is a highly reactive chemical and therefore transporting it over long distance is not only expensive but also increasingly hazardous. For a smaller plant operator, it makes good economic sense to boost the capacity of sulfuric acid by employing oxygen enrichment in the SO2 generation and or oxidation stage.
Oxygen enrichment when done to the SO2 generation stage, not only increases throughput, but also can improve thermal efficiency thereby reducing fuel requirements, increasing SO2 concentration in the process gas stream, and exporting more steam and reducing unit product cost. Replacing some of the combustion/oxidation air with gaseous oxygen not only improves capacity of the furnace but also increases SO2 content of the process gas stream exiting the furnace. Generally downstream equipment such as catalytic converters, waste heat recovery equipment, fans, etc. operate more effectively at higher concentration of SO2 and lower process gas flow rates. Typical sulphuric acid processing equipment has adequate processing capacity to handle 30 to 40% additional SO2 load. In the case when SO2 is arising from a metal roasting furnace, oxygen enrichment not only improves sulfuric acid throughput but also enhances ore processing capacity.
With all these positive aspects of oxygen enrichment with respect to capacity and costing, there is a major down side. Oxygen enrichment produces higher combustion temperatures in the furnace with greater O2 concentration resulting in higher amount of NOx formation. Without addressing issues regarding higher environmental emissions and increased niter content of the product, full potential or benefits of oxygen enrichment can not be achieved. FIG. 1 depicts the difficulty in economically justifying smaller size plants due to longer payback period. However with O2 enrichment, this payback period can be significantly reduced.